1. Field of the Invention
This invention teaches use of an adaptive deconvolution method for enhancing the vertical resolution of an MWD (measurement while drilling) electromagnetic logging tool.
2. Discussion of Related Art
Wireline induction logging tools are well known in the art of studying the formation characteristics of the earth through which a borehole has been drilled. Typically, a sonde, carrying one or more transmitting coils and one or more receiving coils, is lowered into the borehole on the end of a multi-conductor cable. An AC signal, at a frequency on the order of 20 kHz excites the transmitter coil. Radiation from the transmitter generates an electromagnetic field in the formation which, in turn, induces a flow of eddy currents therein. Variations in the magnitude of the eddy currents due to variations in the formation conductivity (or its inverse, resistivity) are detected by the receivers. The magnitude of formation conductivity is diagnostic of certain parameters of the earth layers that were penetrated by the borehole. Although other types of tools are known for measuring resistivity, induction tools are preferred in many cases for operational reasons.
A quantitative measure of the conductivity is determined by measuring the value of the voltage induced in the receiver coil that is in-phase with the transmitter current (the real or R component). The real signal is a nonlinear function of conductivity. A quadrature component (X-component) signal can also be measured and be combined with the real component such that the resulting value is a linear function of conductivity.
The term "vertical geometric factor" (VGF), or the impulse response is used by those skilled in the art to describe the response of an induction tool to a thin conductive layer. The impulse response of a typical induction tool in a homogeneous formation is a curve that has a main lobe of finite width that spans a length of the borehole and an amplitude that is an inverse non-linear function of the formation conductivity (N.B. the unit of measurement for conductivity is mho and for borehole applications, 0.001 mho or mmho. Some authors use units of resistivity which are measured in ohm-meters. The terms are often used interchangeably). Although most of the signal originates from the main lobe, unwanted side lobes of non-zero amplitudes extend vertically above and beneath the main lobe.
In thin beds of low conductivity, the unwanted contribution of the side lobes from adjacent beds that have higher conductivity, will cause the thin-bed measurements to be too high. That error is called the "shoulder effect".
As before stated, the magnitude of impulse response of the induction logging tool is an inverse function of formation conductivity but that function is non-linear. The non-linearity is referred to as the "skin effect". The magnitude of the skin effect is also a complex function of the system operating frequency and coil separation.
Various methods have been used in the prior-art borehole logging sondes to counteract the problems cited. For example, see U.S. Pat. No. 4,471,436, issued Sep. 11, 1984 to R. T. Schaefer et al. for PHASOR PROCESSING OF INDUCTION LOGS INCLUDING SHOULDER AND SKIN EFFECTS. Shoulder effect is reduced by generating a spatial deconvolution filter which sharpens the main lobe and reduces the side lobes when the filter is convolved with the VGF. The skin effect is reduced by filtering the quadrature phase component measurements according to a non-linear spatial filtering function to obtain a correction representative of the change in sonde response function as a function of the formation conductivity. The correction component measurements are then summed with the processed in-phase component measurement to produce a processed log measurement sans the unwanted side-lobe contributions.
There are certain technical differences between wireline logging tools as described above and tools used in measurement-while-drilling (MWD). In the latter case, the transmitter and receiving coils of the tool are mounted on a highly-conductive metal mandrel to withstand the drilling stresses during operation. The physical configuration of the conductive mandrel and the transmitter/receiver coils requires that the coils be excited at a frequency on the order of MHz. This tool is thus also named as a propagated wave resistivity tool. Whereas the prior-art wireline tools depended upon a combination of the real and quadrature components to derive apparent conductivity, the propagated wave resistivity tool of this invention relies on the phase difference between the measured signals in the two receiver coils. Therefore, the way of evaluating the VGF impulse response function for MWD is different from that for wireline induction application. See for example MWD Resistivity Tool Response In A Layered Medium, by Q. Zhou et al., Geophysics, November 1991, pp. 1738-1748. Also Geometric Factor and Adaptive Deconvolution of MWD-PWR Tools, by Q. Zhou et al., The Log Analyst, July-August, 1992 pp. 390-398.